Resolve SDP configuration library etcd client issues

Presented at System Demo 18.6 ( slides ) The feature has been down-scoped to the implementation of primitive methods on the backend code as well as the full implementation of the Etcd3Transaction class. Work on the Watcher and final integration is moved to next PI. AC1: Spikes carried out to familiarize the team with the following (documentation on Confluence): gRPC: https://confluence.skatelescope.org/display/SE/Investigation+of+gRPC python-etcd3: https://confluence.skatelescope.org/display/SE/Investigation+of+python-etcd3+client+package existing Configuration Library code: https://confluence.skatelescope.org/display/SE/SDP+Configuration+Library A set of abstract classes have been created to formalize the backend API ([MR52| https://gitlab.com/ska-telescope/sdp/ska-sdp-config/-/merge_requests/52 ]) Implementation of primitives using the python-etcd3 client package: MR53 , MR59 , MR61 , MR62 , MR63 Implementation of the Etcd3Transaction class: MR58 , MR59 , MR61 , MR63 At the end of the PI, all original tests were ran with the new code bugs have been fixed: MR65 Additional work related to revisions and leases: MR56 and https://confluence.skatelescope.org/display/SE/Investigation+of+%27alive%27+method A performance test was developed to prove that the new client packages brings improvements to SDP performance as expected, even with the current partial implementation. Results are documented on Confluence: https://confluence.skatelescope.org/display/SE/Performance+Test+on+new+etcd+backend   AC2: Not achieved; because we have only implemented part of the new backend, we have not yet integrated the updates with SDP and the processing scripts.